OK - I seem to have found the magic formula (not immediately apparent in help/howto): With source client having granted Friend Slot, now ensure that client is in your friends list, and with a right-click on slow/stalled transfer Enable Friend Slot at your end!
It would also seem that the Friend Slot as granted from remote client applies to any/all requested files (heart in icon), but each must be enabled at your end.
Something that could be made more obvious? ...